To alleviate an energy crisis caused by insufficient energy supply due to a continually increased energy demand and to mitigate influences on the ecology of the earth associated with conventional power generation from fossil fuels or nuclear power, there is a growing consensus on the use of sunlight as an alternative energy source since the solar energy is unlimited, clean, and ubiquitous.
Photovoltaic (PV) solar cells make it possible to convert the sunlight into electricity. However, a wide use of solar power generation depends on technology advances that lead to higher efficiency, satisfied reliability, and lower cost, thus making it more competitive with the conventional power generation methods. One of the most important and costly aspects of solar cell manufacture is the way of contact metallization (also called “electrode”), which provides electrically conducting paths on the surface of cell to collect and transfer photo converted charges to an external circuit, thus generating useful electrical energy. The reliability and efficiency of solar cells are largely affected by the nature and quality of the metal contacts.
Crystal silicon solar cells constitute an important technology in production. Crystalline solar cells are fabricated using either mono-crystalline silicon or multi-crystalline silicon as substrates. These substrates are commonly modified with a dopant of either positive or negative conductivity type, and are on the order of 50-500 microns in thickness. The surface of the substrate, such as a wafer, intended to face incident light, is designated as the front surface and the surface opposite the front surface is referred to as the back surface. By convention, positively doped silicon is commonly designated as “p”, where holes are the majority electrical carriers. Negatively doped silicon is designated as “n” where electrons are the majority electrical carrier. The key to the operation of a photovoltaic cell is the creation of a p-n junction, usually formed by further doping a thin layer at the front surface of the silicon substrate. Such a layer is commonly referred to as the emitter layer, while the bulk silicon is referred to as the absorber layer. The emitter may be either p-doped or n-doped depending on the configuration of the device.
Electrodes are formed on the front and back of crystalline silicon PV devices. Electrodes on the front are typically formed using metal pastes, composed primarily of silver and silver alloys. These electrodes are deposited in arrays on the front surface of the device using techniques such as screen printing. The substrate is typically the silicon solar cell covered by a dielectric film, typically silicon nitride, that serves as an antireflective coating. Electrodes on the front or back side of the device have the requirement of high conductivity and low contact resistance.
Currently, there is a global trend in the PV industry toward developing thin film solar cells. The reason is that wafer based solar cells are difficult to mass-produce and cannot be provided inexpensively, as they require much time and energy for crystal growth and complex subsequent manufacturing steps. Amorphous silicon type solar cell is expected as one of the most promising thin film solar cells, because it has excellent characteristics such as the operability with thin films due to its great light absorption coefficient and the easy formation in large area on a low-cost or flexible substrate through relatively simple manufacturing steps.
The photoelectric conversion layer in thin film solar cells contains at least one or more p-i-n junctions and the stack of the active layer is normally microns thick, while in conventional wafer based solar cells, the active layer is typically hundreds of microns thick. Therefore, the sheet resistance of the active layer in thin film solar cells is much higher than that in wafer based cells. The high sheet resistance of the active layer retards lateral charge transfer during charge collection by front electrode. While, an increase in the density of grid lines on the sun-facing front face of the solar cell will deteriorate the shading effect on the photovoltaic junction and reduce cell output. To compensate this drawback of thin film active layer, the front electrode of thin film solar cells is generally a transparent conductive oxide (TCO), such as indium tin oxide (ITO) or zinc oxide, which enables incident light to reach the light absorbing material and serves as an ohmic contact to collect electrical charges converted there from the light radiation. The TCO also acts as an anti-reflective coating (ARC) layer. Since the resistance of TCO is intrinsically high compared with those of thick film made charge collectors, TCO must be made hundreds of nanometers to microns thick in order to create an electrode layer with a sufficient conductivity. Even though, metal grid lines are still required to be added on the TCO surface to further assist in charge collecting. An intimate contact between metal in the grid lines and TCO surface is highly desired to ensure the efficiency of the charge collecting.
Several methods are known for electrode formation. Vapor deposition or sputtering to form a conductive metal can form electrodes, but suffers from the drawback that these methods are not cost effective on large-area cell devices. One method for forming electrode on thin film and silicon wafer based PV solar cells is using screen printed thick film technology.
The quality of amorphous silicon can be easily damaged when processing temperatures subsequent to the formation of the active layers is at or above 250° C. For this reason, polymer thick film (PTF) method is desirable for forming metal grid lines. PTF pastes may contain metal particles, polymer resins, and other additives. The pastes are deposited on the TCO surface by screen printing into a grid line pattern and then curing at a relatively low temperature, such as below 250° C. After curing, metal particles are physically connected each other and fixed by a polymer matrix, thus forming a conductive film. The polymer resins or binders also provide an adhesion between PTF and TCO. However, charge collecting electrodes manufactured by these known methods suffer from the drawback of a decrease in conversion efficiency and are not satisfactory in the reliability in the prolonged use as compared with those electrodes made of metal thick film method. The drawbacks for electrodes formed from these methods include the following:                1) The resistivity of such types of collecting electrodes (approximately 30 to 50 μΩ-cm) is significantly higher than that made by metal thick film on c-Si based cells (5 to 8 μΩ-cm), leading to an increase in Joule loss and this increases the loss of conversion efficiency.        2) Polymer binders do not act as barriers to oxygen. The resistivity of the electrode increases with time due to a continual oxidation on the metal particle surface, which largely reduces the electrode durability against temperature and humidity.        3) The curing time of PTF is normally long (e.g. 30 minutes to one hour or more), which largely increases the process cycle time.        4) The solderability of the formed PTF electrode is normally poor due to insufficient and embedded metal particles.        5) Some material in PTF, such as a polymer binder, can optically degrade upon irradiation with light to cause breaking of the electrode and peeling off of the electrode from TCO surface, thus making the electrode to lose its function.        6) It is the surface oxidation of metal particles in the existing PTF technology that inhibits the use of most metals as conductive fillers. Only noble metals, such as silver, are widely used as a conductive filler material in PTF, which not only increases the material cost but also brings a silver migration problem.        
Generally speaking, copper is more desirable than silver as a printed conductor due to its low material cost and favorable electrical properties, such as higher electro migration resistance and lower self diffusivity. However, copper differs from silver because it normally has a relatively thick and non conductive native oxide layer on the surface and it has a high oxidation potential, which are significant technical challenges associated with switching silver by copper for most of printed electronics applications. Therefore, the use of copper thick film for front-side contacts on thin film PV solar cells is not generally known.
Following efforts have been made by previous researchers to solve the oxidation induced degradation of electrical performance. Dielectric encapsulates have been applied to adhesives to form an oxidation barrier, but this only retards rather than prevents oxidation. Oxide-reducing agents have been added to adhesives and this approach is only effective over a short period of time. Acid anhydride epoxy hardener has also been used. The epoxy can initially reduce surface oxides, but continual aging causes the oxide to reform. Using a metal alloy powder with a low melting point as a binder to bond Ag particles into a network structure in the conductive adhesive has been pursued. However during soldering, the formed network structure can be destroyed and metal alloys have an intrinsically, low electrical conductivity.
As described above, typical PTF conducting materials are made of individual particles which are physically connected each other. In contrast, PARMOD® materials provide “chemical welding” of pure metals including copper, thus forming electrical conductors made of a continues network structure. These materials are disclosed in, for example, in U.S. Pat. No. 5,882,722, U.S. Pat. No. 6,036,889, U.S. Pat. No. 6,143,356, U.S. Pat. No. 6,153,348, U.S. Pat. No. 6,274,412 B1, U.S. Pat. No. 6,379,745 B1, which are hereby incorporated by reference in their entirety. In addition, PARMOD® materials can be consolidated at relatively low temperatures (e.g., 220° C.) in a forming gas environment (e.g., 5% H2 in N2) with an electrical resistivity better than that of Ag PTF (e.g., 21 μΩ-cm for PARMOD® copper versus 30 to 50 μΩ-cm for Ag PTF).
The PARMOD® compositions generally contain a Reactive Organic Medium (ROM) and metal flakes and/or metal powders, which are blended together with liquid organic vehicles, to produce printable inks or pastes or toners. The ROM consists of either a Metallo-Organic Decomposition (MOD) compound or an organic reagent, which can form such a MOD compound upon heating in the presence of the metal constituents. During heating, individual metal atoms can be generated thermally upon decomposition of the MOD compound, which thus “chemically welds” the flakes and/or powder constituents of the mixture together into a network structure. A carboxylic acid, such as neodecanoic acid, is used in PARMOD® copper composition as a fluxing agent to remove oxides from copper particle surface. This oxide removal forms copper neodecanoate, which acts as a copper MOD compound and generates fresh copper atoms for particle fusion during thermal consolidation.
However, the capability of removing oxides on copper particle surface at low temperatures (e.g. 200° C. or below) by using the PARMOD® compositions is limited, such that a dense particle packing and an efficient particle fusion may be hindered during the consolidation process. As a result, the electrical conductivity and mechanical strength of the sintered copper conductor may not be satisfied. It is desirable to have little or no residual non-conductive species from the oxide removal process. In addition, the oxide removal should be depressed at room temperature to maintain chemical stability and shelf life of the conductive composition. Furthermore, it is desirable to reduce the temperature for copper conversion from copper MOD, thus copper conductor can be formed at low temperatures.
PARMOD® copper conductive material does not adhere well on commonly used substrates, such as polymers, papers, and TCO surfaces. Achieving sufficient adhesion without sacrificing the desired electrical resistivity in the conductive feature is very challenging. For example, carboxylic acids are also very effective crosslinking moieties when present in their reactive form in a composition containing a thermosetting resin, such as an epoxy. Therefore, to use a thermosetting resin in the conductive composition for achieving an adhesion to a substrate, a chemical protection of the carboxylic acid has been applied to ensure chemical stability and prevent premature polymerization. The protection can be achieved by binding the functional group of the carboxylic acid with a chemically or thermally triggered species, so that it becomes reactive only above certain temperatures. However, this approach will reduce the oxide removal capability of the acid and the oxide removal is very critical for particle fusion of non-precious metals, such as copper.
An effort was made by adding a separate adhesive layer to the substrate surface for adequate adhesion of PARMOD® materials to rigid printed circuits. However, because the adhesive layer can infiltrate into the porous metal trace during thermal consolidation, this approach degrades the electrical performance of the PARMOD® compositions. Suitable adhesive coatings are not widely available on substrates of commercial interest, such as paper and polymer based substrates. In addition, coated substrates are generally more expensive than uncoated substrates.
Although attempts, such as US 2004/0144958 A1, which is hereby incorporated by reference in its entirety, have been made to improve adhesion of the PARMOD®conductive materials, a suitable solution to this problem has still not been developed, especially for copper. For example, there was no any result demonstrating electrical performance and adhesion for the polymer-containing PARMOD® copper compositions. In addition, the thermal consolidation temperature for copper was quite high, such as 350° C.
U.S. Pat. No. 6,951,666, which is hereby incorporated by reference in its entirety, describes a screen printable metal precursor composition with a viscosity above 1000 centipoises for fabricating a conductive feature on a substrate. The composition contains a metal precursor compound, a liquid vehicle, metallic particles, and other additives, such as a polymer. During heating, the metal precursor compound will convert to metal thermally or thermal-chemically. However, the U.S. Pat. No. 6,951,666 patent doesn't teach forming non-precious metal conductors, such as copper conductors, having good adherence and low resistivity. Examples of copper compositions given in the U.S. Pat. No. 6,951,666 patent have poor electrical performance. For example as introduced in the U.S. Pat. No. 6,951,666 patent the resistivity of the formed copper after a thermal consolidation at 350° C. is 24 times of bulk copper (example 30).
Accordingly, there is a need in the art to provide compositions comprising metals, particularly non-precious metals such as copper, and methods comprising same, that provide sufficient adhesion of conductive feature on substrates, especially on thermally sensitive substrates such as on TCO surface, and that have improved electrical conductivity and mechanical strength of the formed metal features. In addition, it is desirable that the compositions have chemical stability and extended shelf life.